1. Field of the Invention
The present invention relates to a seat cushion pad supporting construction and, more particularly, to a seat cushion pad supporting construction for, for example, a vehicle seat, which provides seating comfort despite a comparatively thin cushion pad disposed on the supporting construction.
2. Description of the Related Art
A known vehicle seat cushion pad supporting construction comprises a spring deck disposed in or over an opening of a seat cushion base frame and tension springs resiliently connecting the spring deck to the seat cushion base frame. This type of cushion pad supporting construction employing a spring deck is widely used because the cushioning performance of the construction allows use of a relatively thin cushion pad.
Japanese utility model application unexamined publication No. 62-42453 and Japanese utility model application unexamined publication No. 2-31531 disclose vehicle seat cushion supporting constructions. Each construction employs a planar spring deck that includes a generally rectangular boundary frame and a plurality of spring steel wires extending between the right and left side frame portions of the boundary frame. The spring deck is resiliently connected at its right and left side frame portions to the seat cushion base frame by a plurality of tension springs. Although these constructions provide a high degree of static seating comfort, they fail to provide dynamic seating comfort and, particularly, a high degree of seating surface stability under dynamic conditions. For example, the prior constructions fail to provide comfort where substantial vibration, such as pitching or rolling, of the vehicle body changes the load exerted on the seat cushion from the occupant or shifts the center of gravity of the occupant relative to the planar spring deck. In addition, because the spring steel wires extend in the right-to-left directions, the load of an occupant sitting on the seat is unevenly distributed to the spring steel wires; normally, rearwardly-disposed spring steel wires receive larger loads than forwardly-disposed spring steel wires. As a result, rearward portions of the side frame portions receive a large force, and the elastic deformation of the boundary frame is likely to become significantly large. Further, such uneven distribution of load over the spring deck results in high seating surface pressure. Therefore, a relatively thick cushion pad is required to achieve a desirable cushioning performance despite the high seating pressure.
U.S. Pat. No. 3,860,287 discloses a seating construction incorporating a spring deck member including a three-dimensional border strand whose right and left side strand portions are raised higher than front and rear strand portions. Spring steel wires extend between front and rear strand portions of the border strand. While the rear portion of the spring deck is resiliently connected to a rear rail of a seat cushion base frame by means of, for example, tension springs, the front frame portion thereof is rigidly fixed to or connected pivotably (in a substantially vertical direction) to a front rail of the seat cushion base frame. Similar to the above-described constructions, although this construction provides good static seating comfort it fails to provide good seating surface stability under dynamic conditions. More specifically, when the body of an occupant is swung side-to-side during turning or rolling of the vehicle, the raised side frame portions of the boundary frame alternately receive increased loads and thereby elastically bend. Along with elastic deformation of the side frame portions of the boundary frame, the rear frame portion of the boundary frame also bends elastically, thus deforming and twisting a central seating portion of the spring deck.
Japanese Utility model unexamined application Publication 56-36261 discloses another seat cushion supporting construction employing a spring deck comprising a generally rectangular frame. Each of a plurality of frame cords is connected at one end to a front portion of the frame and, at the other end, to a rear portion of the frame. The front portion is rigidly connected to hooks that are fastened to a front of a seat frame. The rear portion of the frame is connected by coil springs to the rear of the seat frame. However, there is no disclosure for the prevention of twisting movement of the frame and, in particular, a rear portion of the frame, which occurs under dynamic seating conditions. Moreover, there is no provision for a rear frame member that includes a rigidity that resists bending during application of a load.
Japanese utility model application unexamined publication No. 2-31531 discloses yet another seat cushion supporting construction employing a spring deck that includes spring steel wires extending in right-to-left directions in the boundary frame. The spring deck is resiliently connected at its rear edge to a seat cushion base frame and is non-resiliently connected at its front edge to the seat cushion base frame by a plurality of hooks. However, because the hooks do not sufficiently restrict the front edge of the spring deck from moving right-to-left or from rolling about a front-to-rear axis, this construction fails to provide good seating surface stability under dynamic conditions.